We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 10^{12} cm^{-2} per graphene sheet. The most highly-ordered sample exhibits Shubnikov - de Haas oscillations which correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nano-patterned epitaxial graphene (NPEG), with the potential for large-scale integration.

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics

The spin-orbit coupling in graphene induces spectral gaps at the high-symmetry points. The relevant gap at the Γ point is similar to the splitting of the p orbitals in the carbon atom, being roughly 8.5 meV. The splitting at the K point is orders of magnitude smaller. Earlier tight-binding theories indicated the value of this intrinsic gap of 1 μeV, based on the σ−π coupling. All-electron first-principles calculations give much higher values, between 25 and 50 μeV, due to the presence of the orbitals of the d symmetry in the Bloch states at K. A realistic multiband tight-binding model is presented to explain the effects the d orbitals play in the spin-orbit coupling at K. The π−σ coupling is found irrelevant to the value of the intrinsic spin-orbit-induced gap. On the other hand, the extrinsic spin-orbit coupling (of the Bychkov-Rashba type), appearing in the presence of a transverse electric field, is dominated by the π−σ hybridization, in agreement with previous theories. Tight-binding parameters are obtained by fitting to first-principles calculations, which also provide qualitative support for the model when considering the trends in the spin-orbit-induced gap in graphene under strain. Finally, an effective single-orbital next-nearest-neighbor hopping model accounting for the spin-orbit effects is derived.

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Tight-binding theory of the spin-orbit coupling in graphene structures

The wave nature of electrons in low-dimensional structures manifests itself in conventional electrical measurements as a quantum correction to the classical conductance. This correction comes from the interference of scattered electrons which results in electron localisation and therefore a decrease of the conductance. In graphene, where the charge carriers are chiral and have an additional (Berry) phase of \pi, the quantum interference is expected to lead to anti-localisation: an increase of the conductance accompanied by negative magnetoconductance (a decrease of conductance in magnetic field). Here we observe such negative magnetoconductance which is a direct consequence of the chirality of electrons in graphene. We show that graphene is a unique two-dimensional material in that, depending on experimental conditions, it can demonstrate both localisation and anti-localisation effects. We also show that quantum interference in graphene can survive at unusually high temperatures, up to T~200 K.

Transition between electron localisation and antilocalisation in graphene

Topics in Applied Physics is a well-established series of review books, each of which presents a comprehensive survey of a selected topic within the area of applied physics. Edited and written by leading research scientists in the field concerned, each volume contains review contributions covering the various aspects of the topic. Together these provide an overview of the state of the art in the respective field. Topics in Applied Physics is addressed to all scientists at universities and in industry who wish to obtain an overview and to keep abreast of advances in applied physics. The Managing Editors are open to any suggestions for topics coming from the community of applied physicists no matter what the field and encourage prospective book editors to approach them with ideas. 2015 Impact Factor: 0.889

Topics in applied physics

In addition to possessing unique electrical properties, silicon carbide (SiC) can crystallize in different modifications (polytypes). Having the same chemical nature, SiC polytypes may significantly differ in their electrical parameters. In recent years, the world's interest in the fabrication and study of heteropolytype structures based on silicon carbide has considerably increased. This review considers studies concerned with polytypism in SiC, fabrication technologies of various types of heterostructures constituted by different SiC polytypes and their electrical parameters. It is shown that heterostructures between SiC polytypes may have a better structural perfection than those constituted by semiconductors that differ in chemical nature. A conclusion is made that SiC-based heterostructures are promising for application in modern electronic devices.

https://iopscience.iop.org/article/10.1088/0268-1242/21/6/R01/pdf

Heterojunctions and superlattices based on silicon carbide

Cubic silicon carbide (3C-SiC) epilayers with different double position boundaries (DPBs) and stacking faults (SF) density grown on n-6H-SiC Lely substrates by vacuum sublimation epitaxy were characterized by optical microscopy after oxidation in dry O 2 at 1100 °C and X-ray topography. Using the methods of electron beam-induced current (EBIC) and secondary electrons (SE), the intermediate 6H-SiC layer at the interface between 3C-SiC epilayer with high DPBs and SF density and 6H-SiC substrate was detected. Triangular shape regions crossing the upper 3C-SiC epilayer represent local p-n junctions and look like the cross-section transmission electron microscopy (TEM) images of structures with SF.

Characterization of 3C-SiC epilayers grown on 6H-SiC substrates by vacuum sublimation

https://iopscience.iop.org/article/10.1088/1361-6641/ab4b05/pdf

MoSe2/graphene/6H-SiC heterojunctions: energy band diagram and photodegradation

The effect of 8 MeV proton irradiation on n-3C-SiC epitaxial layers grown by sublimation on semi-insulating 4H-SiC substrates has been studied. Changes in sample parameters were recorded using the Hall-effect method and judged from photoluminescence spectra. It was found that the carrier removal rate (Vd) in 3C-SiC is ~100 cm−1, which is close to Vd in 4H-SiC. Compared with 4H and 6H silicon carbide, no significant increase in the intensity of the so-called defect-related photoluminescence was observed. An assumption is made that radiation-induced compensation processes in 3C-SiC are affected by structural defects (twin boundaries), which are always present in epitaxial cubic silicon carbide layers grown on substrates of the hexagonal polytypes.

Radiation Defects in Heterostructures 3C-SiC/4H-SiC

This work presents the results of an investigation of thin layers of MoSe2 on graphene by Scanning Probe Microscopy (SPM) methods. Dependences of the surface potential and work function on the number of monolayers of the structure are presented. The dependence of the surface potential on air humidity is shown. The band structure and doping of the MoSe2 monolayer on graphene was determined. These data can be important for detecting of the number of MoSe2 layers and for designing nanodevices, because the surface potential has a strong influence on the operation of such devices.

https://iopscience.iop.org/article/10.1088/1742-6596/1124/8/081031/pdf

Kelvin probe microscopy of MoSe2 monolayers on graphene

In this paper, we studied cobalt intercalation of single-layer graphene grown on the 4H-SiC(0001) polytype. The experiments were carried out in situ under ultrahigh vacuum conditions by high energy resolution photoelectron spectroscopy using synchrotron radiation and low energy electron diffraction. The nominal thicknesses of the deposited cobalt layers varied in the range of 0.2–5 nm, while the sample temperature was varied from room temperature to 800°C. Unlike Fe films, the annealing of Co films deposited on graphene at room temperature is shown to not intercalate graphene by cobalt. The formation of the graphene–cobalt–SiC intercalation system was detected upon deposition of Co atoms on samples heated to temperatures of above ~400°C. Cobalt films with a thickness up to 2 nm under graphene are formed using this method, and they are shown to be magnetized along the surface at thicknesses of greater than 1.3 nm. Graphene intercalation by cobalt was found to be accompanied by the chemical interaction of Co atoms with silicon carbide leading to the synthesis of cobalt silicides. At temperatures of above 500°C, the growth of cobalt films under graphene is limited by the diffusion of Co atoms into the bulk of silicon carbide.

Alexander A. Lebedev

Papers

Characterization of 3C-SiC epilayers grown on 6H-SiC substrates by vacuum sublimation

MoSe2/graphene/6H-SiC heterojunctions: energy band diagram and photodegradation

Radiation Defects in Heterostructures 3C-SiC/4H-SiC

Kelvin probe microscopy of MoSe2 monolayers on graphene

Cobalt Intercalation of Graphene on Silicon Carbide